(1) Field of the Invention
The invention of the present application relates to a main bearing for supporting a crankshaft of an internal combustion engine.
(2) Description of Related Art
A crankshaft of an internal combustion engine is supported at a journal section thereof by a cylinder block lower part of the internal combustion engine via a main bearing made of a pair of half bearings. For the main bearing, a lubricating oil that is discharged by an oil pump is fed into a lubricating oil groove that is formed along an inner circumferential surface of the main bearing through a through-port that is formed in a wall of the main bearing from an oil gallery that is formed in a cylinder block wall. Further, a first lubricating oil path is formed by being penetrated in a diameter direction of the journal section, and both end openings of the first lubricating oil path communicate with the lubricating oil groove of the main bearing. Furthermore, a second lubricating oil path passing through a crank arm section is formed by branching from the first lubricating oil path in the journal section, and the second lubricating oil path communicates with a third lubricating oil path that is formed by being penetrated in a diameter direction of a crankpin. In this manner, the lubricating oil which is fed into the lubricating oil groove formed on the inner circumferential surface of the main bearing through the through-port from the oil gallery in the cylinder block wall passes through the first lubricating oil path, the second lubricating oil path and the third lubricating oil path, and is supplied between a sliding surface of the crankpin and a connecting rod bearing from a discharge port that is opened in a terminal end of the third lubricating oil path (for example, see JP-A-8-277831).
The lubricating oil which is fed to the lubricating oil groove of the main bearing from the oil gallery in the cylinder block wall is likely to be accompanied by residual foreign matters that are generated at a time of machining of respective components, for example. The foreign matters are likely to damage sliding surfaces between the journal section and the main bearing and sliding surfaces between the crankpin and the connecting rod bearing, and therefore, need to be discharged to an outside quickly from the flow of the lubricating oil.
Consequently, there has been conventionally provided a main bearing for supporting a crankshaft of an internal combustion engine, wherein an inner circumferential surface of each of semi-cylindrical bearings that configure the main bearing is formed of two kinds of circular arcs with different curvatures, namely, a first curved surface (a main cylinder portion) including a circumferential central portion and a second curved surface (a crush relief portion), an axial groove is formed through an entire length in a width direction of both butting end surfaces of the semi-cylindrical bearings, the second curved surface is formed in a range of 20° to 50° as a circumferential angle (θ) measured from a circumferential end surface of each of the semi-cylindrical bearings, and a number of fine circumferential grooves are formed in the second curved surface (FIGS. 1 and 5 of JP-A-2011-58568), so that foreign matters that accompany the lubricating oil and enter a lubricating oil groove of one of the semi-cylindrical bearings are fed to and dispersed in a gap formed by the second curved surface and a surface of the crankshaft, are thereafter discharged to an outside of the main bearing from the axial groove, and are prevented from reaching an inner circumferential surface position of the other semi-cylindrical bearing and being locally embedded and accumulated in the same inner circumferential surface (paragraph 0011 of JP-A-2011-58568).
Further, there has been proposed a sliding bearing in which one circumferential end portion of a lubricating oil groove formed along an inner circumferential surface of one of semi-cylindrical bearings constituting a main bearing communicates with a crush relief and a chamfer, and the other circumferential end portion does not communicate with a crush relief, for ensuring compatibility of reduction of the amount of a lubricating oil flowing out of an oil clearance of the bearing, and inhibition of occurrence of an damage due to a foreign matter (the abstract of JP-A-2008-95858).
However, a bearing gap between the inner circumferential surface (the first curved surface and the second curved surface) of a sliding bearing (half bearing) and a journal surface has been set to be narrow conventionally. For example, in the case of the main bearing disclosed in JP-A-2011-58568, the formation range of the second curved surface is set to be large possibly, namely, at 20° or more at a minimum toward the central potion side from the circumferential end surface of the half bearing, and the difference between a bearing wall thickness W1 in the circumferential central portion and a bearing wall thickness W2 at the circumferential end portion of a half bearing 10 is 5 to 30 μm. Therefore, the main bearing has the following problems.
(1) On an upstream side of the gap region, a gap between the second curved surface on a circumferential central portion side of the half bearing from the circumferential central portion of the second curved surface and the journal section surface of the crankshaft becomes too narrow with respect to the sizes of the foreign matters that enter the oil groove. Further, “the pressure of the oil which accompanies the journal surface and flows to the circumferential end portion side in the gap between the second curved surface and the journal surface>the pressure of the oil in the oil groove” is established, and the foreign matters are difficult to disperse while flowing to the gap between the second curved surface and the journal surface from the oil groove. Further, (a few of) foreign matters that are discharged into the gap are pushed into the second curved surface by the journal section surface of the crankshaft and accumulate.
(2) Further, in the formation range of the second curved surface, an increasing rate of the gap per unit circumferential angle toward the circumferential end portion side from the circumferential central portion side of the half bearing is small, and therefore, on a downstream side of the gap region, the oil with a high pressure in the gap between the second curved surface on the circumferential central portion side of the half bearing and the journal surface (namely, on an upstream side of the gap region) accompanies the journal surface and is fed to the gap between the second curved surface on the circumferential end portion side of the half bearing and the journal surface (namely, the downstream side of the gap region). Thereby, on the downstream side of the gap region, “the pressure of the oil which accompanies the journal surface and flows to the circumferential end portion side in the gap between the second curved surface and the journal surface>the pressure of the oil in the oil groove” also tends to be established. Consequently, as shown in FIG. 14A, among the foreign matters in the oil groove, only a few of foreign matters flow to the gap between the second curved surface on the circumferential end portion side of the half bearing and the surface of the journal section, and most of the foreign matters advance to the circumferential end portion side of the oil groove, and easily enter the inner circumferential surface of the other half bearing. Furthermore, by the pressure of the oil which accompanies the journal surface and flows to the circumferential end portion side in the gap between the second curved surface and the journal surface as described above, a few of foreign matters that flow out to the gap also pass over the axial groove and are easily forced to flow to the inner circumferential surface of the other half bearing, as shown in FIGS. 14A and 14B.
Also, the conventional sliding bearing disclosed in JP-A-2008-95858 has the above problem regarding discharge of the foreign matters. Further, since this conventional sliding bearing is configured so that one of the circumferential end portions of the lubricating oil groove does not communicate with the crush relief, a state where an opening of an inner oil path on a journal surface does not communicate with the lubrication oil groove is generated, and the amount of the lubricating oil tends to be insufficient in a circumferential end region of the lubricating oil groove on a rear side in a rotation direction of the crankshaft, so that supply of the lubricating oil toward a crankpin portion side becomes intermittent.
Meanwhile, if a partial groove communicating with the lubricating oil groove is formed in the other half bearing for constant oil supply toward the crankpin portion side as shown in FIG. 42 of JP-A-2008-95858, most of the foreign matters are forwarded to the other half bearing by being guided by the partial groove, and tend to concentrate and be accumulated on a sliding surface in front of the partial groove.